Many useful medical signal instruments require processing of voltages resulting from muscle or nerve activity within a living being. For example, an electrocardiograph (ECG) measures voltages on the surface of the body that originate from nerve and muscle activity involved in the pumping action of heart muscles. Similarly, electromyographs measure voltages on the surface of the body that originate from muscle activity. Electroencephalographs measure voltages on the surface of the skull that result from aggregate chemical and neuron activity in the brain. Physiological signal measurements typically require surface electrodes, usually small conductive discs or pads attached to the skin with a conductive gel.
In addition to voltage measurements, some physiological information of interest may be detected by measurement of impedance. For example, the resistance of the chest varies with the volume of contained air. Therefore, chest impedance can be used to measure respiration. Another use of impedance measurement is to detect whether electrodes are adequately attached.
A front end for a signal measurement instrument is the circuitry directly interfacing to the signal of interest. Front ends for physiological signal measurements require high input impedance, low noise, high gain amplifiers. Contact resistance between an electrode and the body surface can be as high as 1 Megohm. Signal levels can vary from a few microvolts for electroencephalographic activity to a few millivolts for muscle activity.
Usually, in physiological signal measurement, the signals of interest are several orders of magnitude smaller than electrical noise levels. Metal electrodes in contact with conductive gels and natural body electrolytes create battery-like electrochemical processes which can produce DC offset voltages on the order of 100 mV. This offset may change with movement, for example respiration. Amplifiers also typically have some DC offset voltage at the input as well as some very low frequency noise (1/f noise). 50 Hz or 60 Hz power lines can produce voltages on the order of 20V p-p on the surface of the body. Fluorescent lights can create 100 Hz or 120 Hz bursts of higher frequency noise. Other sources of noise include cardiac pacemakers and electronic scalpels. Therefore, extracting a signal from noise is a requirement for physiological signal instrumentation.
Of particular interest in an ECG application is measurement of late potentials. These are very low level voltages following the R-wave. Low noise is essential to discriminate subtle changes in these low level signals.
Typically, the signals of biomedical interest are relatively low frequency. For example, the frequencies of interest in ECG's are less than 500 Hz. Therefore, low pass filters may be used to remove some noise. In addition, analog offset subtraction or high-pass filtering is needed to remove DC offset voltages.
Some noise such as 50 Hz or 60 Hz power line noise is mostly common mode noise (same magnitude over the entire body surface). Physiological signals are typically measured differentially (voltage of one electrode relative to another electrode) so that common mode noise voltages can be eliminated by using differential amplifiers with high common mode rejection. An alternative way to reduce common mode signals is to subtract the common mode signal at the patient. This may be accomplished by measuring the common mode (summation of signals) and driving the patient with an opposite polarity voltage. For example, see Bruce B. Winter and John G. Webster, Driven-Right-Leg Circuit Design, IEEE Transactions on Biomedical Engineering, Vol. BME-30, No. 1, Jan 1983, pp 62-65.
Another requirement for physiological instrumentation is safety; protection of the patient from electrical shock. Any circuitry directly connected to a patient must be battery powered or isolated from normal AC power sources. In addition, the currents in any signals used for common mode offset or impedance measurement must be limited. For example, see American National Standard for Diagnostic Electrocardiographic Devices, ANSI/AAMI EC11-1982 (available from the Association for the Advancement of Medical Instrumentation).
In addition to protection of the patient, there are also requirements for protection of the instrumentation input circuitry. For example, if the heart stops beating, a common procedure is to apply a large voltage pulse (on the order of 5 KV) to synchronize the heart muscles (defibrillation). An ECG front end may be connected to the patient during defibrillation. In an emergency, where there is no time to attach normal ECG electrodes, the defibrillator paddles may also be connected directly to an ECG front end as ECG signal electrodes to provide a "quick look" at the electrocardiogram before defibrillation. The input circuitry of an ECG front end must be able to withstand defibrillator voltage pulses.
In addition to analog filtering and amplification, typical physiological signal instrumentation will include analog to digital conversion for further processing by an internal microprocessor or an external computer. Some multi-channel analog to digital conversion designs use a sample and hold circuit on each channel and a single high speed analog to digital circuit. Sample and hold circuits sample noise as well as signals. In addition, there may be time offsets between samples from different channels which can create spurious noise when measuring the digital difference between two channels. Continuous analog to digital conversion is useful to provide additional noise averaging and to avoid sample offset problems.
Analog to digital conversion circuits may have offset circuitry to extend the dynamic range. Monitoring an ECG in the presence of pacemakers or defibrillation presents special problems. Any offset circuit must be flexible or "intelligent" enough to ignore a single defibrillation pulse or periodic signals such as pacemaker pulses.
A typical modern instrument will also have digital control of various functions. Therefore, physiological signal instrumentation also requires a high frequency clocking signal and various digital circuitry. Digital clocking signals and circuitry create noise which may interfere with nearby analog circuitry. Therefore, additional noise reduction may be required to suppress digital noise. Common-mode noise suppression is especially important.
There is a need in the biomedical instrumentation field for large-scale integrated circuits having all the functionality described above: analog amplification with low noise and high input impedance, input protection, low pass filtering, DC offset subtraction, external common mode subtraction, internal common mode noise reduction, analog to digital conversion, impedance measurement, safety features (isolation from AC power and current limitation), plus additional calibration, configuration flexibility and convenience features.